Magnetic transducer technology has gained wide acceptance for measuring torque in shafts or other parts for transmitting torque. Magnetic-based torque transducers have found application in non-contacting torque sensors particularly for a shaft which rotates about its longitudinal axis. One or more magnetic regions, used as one or more transducer elements, are created in or on the shaft to emanate a torque-dependent magnetic field component external to the shaft which is detected by a sensor arrangement that is not in contact with the shaft.
The following description will be given in the context of shafts but will be understood to apply to measuring torque in parts in general, unless the context otherwise requires.
One class of magnetic region used as a transducer element in torque transducers is self-excited in that it is a region of permanent or stored magnetization which emanates an external torque-dependent field. The transducer region is sometimes referred to as “encoded” in that a predetermined configuration of magnetization is stored in it.
A transducer element may be created in a region of stored or permanent (remanent) magnetization in a ferromagnetic integral region of the shaft or part. Transducer elements of this kind are disclosed in published PCT applications WO99/56099, WO01/13081 and WO01/79801. The stored magnetization may be of the kind known as circumferential in an integral region of a ferromagnetic shaft as disclosed in WO99/56099 or it may be a circumferentially magnetized ring secured to the shaft as disclosed in U.S. Pat. No. 5,351,555. Circumferential magnetization forms a closed peripheral loop about the shaft and produces an axially-directed external field in response to applied torque. Another form of stored magnetization is an integral portion of a shaft in which the stored magnetization is in an annulus about the axis of the shaft and is directed longitudinally, that is in the direction of the shaft axis. One kind of longitudinal magnetization is known as circumferential (tangential)-sensing longitudinal magnetization and is disclosed in WO01/13081; another kind is known as profile-shift longitudinal magnetization as disclosed in WO01/79801
All these forms of transducer magnetization are “D.C.” magnetizations in the sense they operate with unipolar magnetic fluxes generated by the remanent magnetism stored in permanent magnets.
The sensor devices used with self-excited transducer elements may be of the Hall effect, magnetoresistive or saturating core type. These sensor-devices are sensitive to orientation. They have an axis of maximum response, and an orthogonal axis of minimum response.
Another class of magnetic transducer region is externally excited by an energized coil wound about the region. One form of externally-excited transducer is the transformer type in which the region couples an excitation winding to a detector winding, the coupling being torque-dependent. For example the permeability of the transducer element may be torque dependent. The transformer-type of transducer is A.C. energized. An example of a transformer-type of transducer is disclosed in EP-A-0321662 in which the transducer regions are specially prepared to have a desired magnetic anisotrophy at the surface.
Under conditions of severe mechanical stress where the torque transmission is subject to blows, impacts or other sharp pulses, the stored magnetization may deteriorate and the transducer operation become unsatisfactory. In addition the torque measurement may need to be done in circumstances where there are interference magnetic fields of a largely unknown and unpredictable type. Such fields are likely to arise in production plants and such like environments. Magnetic field, interference can arise from unipolar fields, noise spikes and “A.C.” fields at the local powerline frequency, among others.
Another form of externally-excited transducer element is disclosed in WOO/127584 in which the magnetization of a region of a shaft arises from energization of one or more coils that are mounted coaxially with a shaft in which an applied torque is to be measured. The coils are axially spaced and define a transducer region therebetween. The coils are connected in series to generate magnetic fields in the same direction in the transducer region between the coils. The coils are energized to induce a longitudinal magnetic field of a given polarity. The longitudinal field in the transducer region is deflected in direction and to an extent dependent on torque applied to the shaft to produce an external circumferential (tangential) magnetic field component that is a function of torque. The axially-directed component of the field is separately detected to provide a reference against which the circumferential component is measured. If the coils are D.C. energized the resultant magnetic field may be interfered with by other local fields, e.g. fields extending along the shaft whose torque is to be measured.
In the torque measurement system disclosed in WO01/27584, the pair of spaced coils is A.C. energized at a frequency selected to be distinguishable from noise frequencies, e.g. mains power frequency, and the sensor output is also detected in a frequency-selective manner. The torque-dependent “A.C.” or alternating polarity (AP) magnetic field component is detected. By using A.C. energization interfering D.C. fields can be discriminated against and the A.C. frequency can be selected to discriminate against interference as the local power line or supply frequency, e.g. 50 or 60 Hz. The detection may be synchronous with the A.C. energization. The external field to be sensed is enhanced by a pair of spaced collars of magnetic material attached to the transducer region to aid the establishing in a recess between the collars of an external component of the longitudinal field in the transducer region. A sensor arrangement responsive to a torque-dependent magnetic field in the circumferential (tangential) arrangement is disposed in the recess.
The just-described transducer has the advantage that the transducer region does not have to be encoded with a stored magnetization. Nonetheless a transducer region has to be defined between a pair of spaced coils. It would be advantageous to provide a transducer assembly in which no encoding is required and which could be realized in compact form and installed at any convenient location on a shaft or other part subject to torque.
The prior art also discloses various magnetic field sensor arrangements using two or more sensor devices to compensate for potential measurement errors and to assist in nullifying the effects of interfering fields. Although small, such sensor arrangements may be difficult to accommodate in situations where space is at a premium.